Water Treatment
What the exam tests
Water Treatment covers the treatment train from raw water to finished water: coagulation, flocculation, sedimentation, filtration, disinfection, and process control. Expect calculation items on chemical dose, filter loading, backwash, and CT, plus rule-based limits for turbidity and disinfection.
Core rules and concepts
- Conventional filtration: combined filter effluent turbidity is generally limited to 0.3 NTU in at least 95% of monthly samples and must never exceed 1 NTU; individual filter effluent has similar limits under the LT1ESWTR.
- CT = disinfectant residual (mg/L) × effective contact time (minutes). Effective time = theoretical time × baffling factor.
- Jar tests set coagulant dose and pH; filter loading rate = flow (gpm) ÷ filter area (ft²).
- Backwash uses expanded bed, air scour where available, and rinse-to-waste; watch filter-to-waste and breakthrough.
Worked example
A plant treats 1.5 MGD through two filters with a total area of 400 ft². Convert flow: 1,500,000 gal/day ÷ 1,440 min/day = 1,041.7 gpm. Loading rate = 1,041.7 ÷ 400 = 2.60 gpm/ft². If the design rate is 3 gpm/ft², the filters are within capacity; if one filter is out of service, the rate doubles to 5.2 gpm/ft² and exceeds design, so flow must be reduced or the spare filter returned to service. Show units and state the assumption about total effective area.
Traps that cost marks
- Using gallons per day directly in gpm/ft² without dividing by 1,440.
- Confusing theoretical detention time with effective CT; baffling factor always reduces CT.
- Reporting a single turbidity grab sample as if it were the monthly 95th-percentile value.
- Forgetting that disinfection credit depends on pH and temperature, not just residual.